Most synchronous rotary machines, particularly, synchronous electric motors, employ either a radial magnet type rotor formed by alternately and contiguously arranging rotor cores serving as yokes, and permanent magnet pieces along a circumferential direction or a surface-mounted magnet type rotor formed by attaching a plurality of permanent magnet pieces on the outer circumference of a cylindrical rotor core.
The plurality of permanent magnets of the latter of these two types of rotors for synchronous rotating machines, in general, are attached adhesively with an adhesive to the substantially cylindrical circumference of the rotor core. The adhesive strength of the adhesive must exceed the resultant centrifugal force that acts on the permanent magnets when the rotor rotates and the magnetic attraction resulting from the interactions of a rotating magnetic field created by the stator and magnetic fields created by the permanent magnets to prevent the separation of the permanent magnets from the circumference of the rotor core even when the rotor rotates for a long time under the action of the centrifugal force and the magnetic attraction. Accordingly, to ensure that the permanent magnets are securely held on the rotor, the bottom portions of the permanent magnets are partly sunk in recesses formed in the outer circumference of the rotor core so that the permanent magnets are held at their side surfaces by the rotor core when adhesively attaching the permanent magnets to the outer circumference of the rotor core.
The prior art will be described hereunder with reference to the accompanying drawings.
FIGS. 9A and 9B show the construction of a typical surface-mounted magnet type rotor for a conventional synchronous electric motor, in which permanent magnets are held securely on the rotor core solely by the adhesive strength of an adhesive. FIGS. 9A and 9B are a front view and a side view, respectively, of the surface-mounted magnet type rotor. Referring to FIGS. 9A and 9B, a rotor 11 supported for rotation within a stator 10 with a gap between the outer circumference thereof and the cylindrical inner circumference of the stator 10 comprises a rotor shaft 12, a cylindrical rotor core 13 fixedly mounted on the rotor shaft 12, a plurality of permanent magnets 14 having the shape of a modified octagon attached by adhesive at their bottom surface to the outer circumference of the cylindrical rotor core 13, and end plates 15 attached respectively to the opposite ends of the rotor core 13 to hold the rotor core in place on the rotor shaft 12. As mentioned above, the permanent magnets 14 of this typical surface-mounted magnet type rotor are secured to the rotor core 13 solely by the adhesive strength of the adhesive, which is not a sufficiently effective measure to prevent the separation of the permanent magnets 14 from the rotor core 13.
FIG. 10 is a side view, corresponding to FIG. 9B, of a rotor having permanent magnets partly sunk in recesses formed in the outer circumference of a rotor core. As is obvious from the comparison of the constructions shown in FIGS. 9B and 10, the construction shown in FIG. 10 is more effective in preventing separation than the construction shown in FIG. 9B. As shown in FIG. 10, a rotor core 13 is provided in its outer circumference with axial dovetail grooves 16, and permanent magnets 14a having the shape of a curved plate and formed in a shape complementary to that of the dovetail grooves 16 and having an upper surface, a lower surface of a width greater than that of the upper surface, and side surfaces extending outward so as to approach each other and inserted in the dovetail grooves 16. When assembling the rotor core 13 and the permanent magnets 14a, the permanent magnets 14a need necessarily to be inserted axially of the rotor core 13 in the dovetail grooves 16 and positioned in place with respect to the axial direction.
Accordingly, when as shown in FIG. 11, the rotor is provided with a plurality of permanent magnets 14a divided into a plurality of groups (three groups in FIG. 11) and the groups of permanent magnets 14a are arranged respectively in a plurality of axial divisions (three axial divisions in FIG. 11) on the outer circumference of the rotor core 13 with the angular position of the permanent magnets 14a of each group shifted relative to that of the permanent magnets 14a of other groups to suppress torque ripple, it is impossible to insert the permanent magnets 14a of the middle group axially in the corresponding dovetail grooves 16 of the rotor core 13.